1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to wireless local area network (LAN) communications, and more particularly, to wireless LAN (WLAN) communications using an improved carrier sensing mechanism.
2. Description of the Related Art
Recently, there is an increasing demand for ultra high-speed communication networks due to widespread public use of the Internet and a rapid increase in the amount of available multimedia data. Since LANs emerged in the late 1980s, the data transmission rate over the Internet has drastically increased from about 1 Mbps to about 100 Mbps. Thus, high-speed Ethernet transmission has gained popularity and wide spread use. Currently, intensive research into a gigabit speed Ethernet is under way. An increasing interest in the wireless network connection and communication has triggered research into and development of WLANs, greatly increasing availability of WLANs to consumers. Although use of WLANs may reduce performance in view of lower transmission rate and poorer stability as compared to wired LANs, WLANs have various advantages, including wireless networking capability, greater mobility and so on. Accordingly, WLAN markets have been gradually growing.
Due to the need for a greater transmission rate and the development of wireless transmission technology, the initial IEEE 802.11 standard, which specifies a 1 to 2 Mbps transfer rate, has evolved into advanced standards including 802.11b and 802.11a. Currently, a new IEEE standard, 802.11g, is being discussed by the Standardization Conference groups. The IEEE 802.11g standard, which delivers a 6 to 54 Mbps transmission rate in the 56 GHz-National Information Infrastructure (NII) band, uses orthogonal frequency division multiplexing (OFDM) as transmission technology. With an increasing public interest in OFDM transmission and use of a 5 GHz band, much greater attention is been paid to the OFDM than other wireless standards.
Recently, wireless Internet services using WLAN, so-called “Nespot,” have been launched and offered by Korea Telecommunication (KT) Corporation of Korea. Nespot services allow access to the Internet using a WLAN according to IEEE 802.11b, commonly called Wi-Fi representing wireless fidelity. Communication standards for wireless data communication systems, which have been completed and promulgated or are being researched and discussed, include Wide Code Division Multiple Access (WCDMA), IEEE 802.11x, Bluetooth, IEEE 802.15.3, etc., which are known as 3rd Generation (3G) communication standards. The most widely known, cheapest wireless data communication standard is IEEE 802.11b, a series of IEEE 802.11x. An IEEE 802.11b WLAN standard delivers data transmission at a maximum rate of 11 Mbps and utilizes the 2.4 GHz-Industrial, Scientific, and Medical (ISM) band, which can be used at below a predetermined electric field without permission. With the recent widespread use of the IEEE 802.11a WLAN standard, which delivers a maximum data rate of 54 Mbps in the 5 GHz band by using OFDM, IEEE 802.11g developed as an extension to the IEEE 802.11a for data transmission in the 2.4 GHz band using OFDM is intensively being researched.
The Ethernet and the WLAN, which are currently being widely used, both utilize a carrier sensing multiple access (CSMA) method. According to the CSMA method, it is determined whether a channel is in use or not in use. If the channel is not in use, that is, if the channel is idle, then data is transmitted. If the channel is busy, retransmission of data is attempted after a predetermined period of time. A carrier sensing multiple access with collision detection (CSMA/CD) method, which is an improvement of the CSMA method, is used in a wired LAN, whereas a carrier sensing multiple access with collision avoidance (CSMA/CA) method is used in packet-based wireless data communications. In the CSMA/CD method, a station suspends transmitting signals if a collision is detected during transmission. Compared with the CSMA method, in which it is pre-checked whether a channel is occupied or not before transmitting data, in the CSMA/CD method, the station suspends transmission of signals when a collision is detected during the transmission of signals and transmits a jam signal to another station to inform it of the occurrence of the collision. After the transmission of the jam signal, the station has a random backoff period for delay and restarts transmitting signals. In the CSMA/CD method, the station does not transmit data immediately even after the channel becomes idle and has a random backoff period for a predetermined duration before transmission to avoid collision of signals. If a collision of signals occurs during transmission, the duration of the random backoff period is increased by two times, thereby further lowering a probability of collision.
As described above, conventionally, a single input single output (SISO) approach has been adopted for WLAN communications based on a CSMA/CA method. That is to say, a station (hereinafter referred to as an “SISO station”) that adopts the SISO approach receives data from and transmits data to a wireless medium using a single antenna. However, in recent years, research on wireless communications based on a multiple input multiple output (MIMO) approach has been vigorously carried out. A station (hereinafter referred to as an “MIMO station”) that adopts the MIMO approach, unlike an SISO station, transmits a plurality of data to a wireless medium via different transmission paths using a plurality of antennas and receives a plurality of data from another MIMO station via different transmission paths using the antennas. Accordingly, an MIMO station achieves higher data rates (data transferring rates) than an SISO station. However, in a WLAN where an MIMO station and an SISO station coexist, the SISO station may not be able to interpret any data transmitted by the MIMO station. Problems that may arise in such a WLAN will now be described in detail with reference to FIGS. 1 through 3.
FIG. 1 is a diagram illustrating the format of an IEEE 802.11a frame.
Referring to FIG. 1, the IEEE 802.11a frame is comprised of a physical layer convergence procedure (PLCP) preamble 110, a signal field 120, and a data field 130.
The PLCP preamble 110 indicates what data will be transmitted on a current physical layer. The signal field 120, which follows the PLCP preamble 110, includes one orthogonal frequency-division multiplexing (OFDM) symbol that is modulated at a lowest data rate using a basic modulation method. The data field 130 includes a plurality of OFDM symbols that are modulated at data rates higher than or equal to the data rate at which the OFDM symbol of the signal field 120 is modulated.
The signal field 120 is comprised of a total of 24 bits. In detail, the first through fourth bits of the signal field 120 constitute a rate field 142, which specifies how and at what coding rate the data field 130 has been modulated. The fifth bit of the signal field 120 is a reserved bit. The sixth through seventeenth bits of the signal field 120 constitute a length field 144, which specifies the length of the IEEE 802.11a frame.
The eighteenth bit of the signal field 120 is a bit used for parity check. The nineteenth through twenty fifth bits of the signal field 120 are tail bits. The length field 144 specifies the number of bytes constituting a media access control (MAC) frame contained in the data field 130. First through sixteenth bits of the data field 130 constitute a service field. The signal field 120 and the service field constitute a PLCP header 140. The data field 130 also includes a PLCP service data unit (PSDU), six tail bits, and pad bits. The PSDU corresponds to an MAC frame, which is comprised of an MAC header, an MAC data field, and a frame check sequence (FCS) used for determining whether the MAC frame is erroneous. The data field 130 may be modulated in various manners and at various coding rates. As described above, information regarding how and at what coding rate the data field 130 has been modulated is included in the rate field 142 of the signal field 120.
FIG. 2 is a diagram illustrating a carrier sensing operation performed in a WLAN.
Two carrier sensing methods, i.e., a physical carrier sensing method and a virtual carrier sensing method, are currently available for WLAN communications. The physical carrier sensing method and the virtual carrier sensing method will now be described in detail with reference to the accompanying drawings. Referring to FIG. 2, a frame 212, which is received by a physical layer 210, is comprised of a PLCP preamble 214, a signal field 216, and a data field 218.
The physical carrier sensing method enables a station to recognize whether signals are transmitted by a wireless medium. In other words, when the PLCP preamble 214 is input to the physical layer 210, the physical layer 210 notifies an MAC layer 220 that it is currently used by transmitting a busy signal to the MAC layer 220, as marked by 222. Thereafter, when the reception of the PLCP preamble 214 is completed, the physical layer 210 notifies the MAC layer 220 that it is idle by transmitting an idle signal 228 to the MAC layer 220.
A physical carrier sensing operation may be performed based on a result of interpreting a length field included in the signal field 216. The virtual carrier sensing method is a method that enables the MAC layer 220 to determine whether a wireless medium is used based on a result of interpreting a duration value, i.e., a network allocation vector (NAV) value, contained in an MAC frame included in the data field 218. Therefore, for a predetermined period of time specified by the duration value, the MAC layer 220 considers that the wireless medium is used. A station can receive the data field 218 and then read the NAV value from the MAC frame included in the received data field 218.
FIG. 3 is a diagram illustrating a conventional method of transmitting frames in a contention period in a WLAN where three MIMO stations, i.e., first through third MIMO stations, and an SISO station coexist.
According to the physical carrier sensing method, stations are prevented from transmitting frames via a wireless channel when frames are transmitted via the wireless channel by other stations. In a contention mode, stations cannot transmit a next frame immediately after the wireless channel becomes empty but are required to wait for a predetermined amount of time called a distributed inter-frame space (DIFS) and random back-off time to obtain the opportunity to transmit a frame via the wireless channel.
Referring to FIG. 3, the first MIMO station obtains the opportunity to transmit data through channel contention and thus transmits a data frame to the second MIMO station. Since the data frame transmitted by the first MIMO station is an MIMO frame, the third MIMO station as well as the second MIMO station can receive it, but the SISO station cannot receive it. Following a short inter-frame space (SIFS) after receiving the data frame transmitted by the first MIMO station, the second MIMO station transmits an acknowledgement (ACK) frame to the first MIMO frame.
Since the SIFS is shorter than the DIFS and the second MIMO station transmits the ACK frame following a short period of time after receiving the data frame transmitted by the first MIMO station, the second and third MIMO stations and the SISO station cannot transmit data until the transmission of the ACK frame is completed. Since the ACK frame is also an MIMO frame, the third MIMO station as well as the first MIMO station can receive it, but the SISO station cannot receive it.
The first through third MIMO stations can set their respective NAV values based on MIMO data that they receive by performing a virtual carrier sensing operation. Accordingly, the first through third MIMO stations can obtain the opportunity to transmit a next frame the DIFS and back-off time 310 after the transmission of the ACK frame is completed.
However, the SISO station cannot receive the MIMO data and thus cannot perform a virtual carrier sensing operation. In other words, while not receiving any data frame, the SISO frame considers that a collision between data frames has occurred. Therefore, the SISO station can obtain the opportunity to transmit a frame following an extended inter-frame space (EIFS) and back-off time 320 after performing a physical carrier sensing operation, and the EIFS is equal to the sum of the SIFS and a predetermined amount of time required for transmitting an ACK frame at a lowest data rate. In other words, the SISO station must wait a long period of time to obtain the opportunity to transmit a frame in an environment where it exists together with the first through third MIMO stations. Thus, the SISO station is in a disadvantageous position in channel contention with the first through third MIMO stations or other new MIMO stations. Therefore, it is necessary to develop a WLAN communication method that can prevent SISO stations from being discriminated against MIMO stations in an environment where they exist together with the MIMO stations.